I know one guy who was paralyzed from waist down after a MC accident. He lets his wife go out and seek sex with other men. He was the last guy I would accuse of being a wimp or submissive. I know there are strict rules between though.
Rules are important to set boundaries and make sure both of you are on the same page. Those rules tend to change with time and experience and trust. If communication is excellent then the couple will usually thrive. We have only had one blow out which was early on, it was a misunderstanding in our communication. It was bad, but we learnt a lot from it. We started with a lot of rules, but pretty much have one rule, no means no. Doesn't matter the reason, if one of us says no, everything stops no questions asked. We trust each other to the point that we know if either of us says no there is a reason. We discuss it later of course.
Your friend's story is similar to how we started. My first marriage was not a good marriage. The final years were sexless, we stayed together for the kids. I vowed I would never let that happen again to me or do that to a partner. It was a very bad situation and divorce was inevitable.
I then met my wife a few years later. Our chemistry was off the charts. 8years in, I then suffered some serious health problems and suddenly sex was out for me. I said to my wife it isn't fair for her. She had a high libido and she deserved better. She is a very good woman. She said no of course. Over time I can see it wearing on her and suggested it again and so we became part of the cuckold community about 10yrs ago. It helped that I like to watch and she likes me watching her. I still had all the emotions desires etc, I just could not act on it a lot of the time due to my health. I did get to engage with many in the community and learnt so much from them.
Fast forward 9 years and my health finally improved to the point we decided to move our relationship to an open relationship and maybe even do some swinging. Although I wont date without my wife with me. Its just a me thing, not a rule or anything.
Anyway, there are lots of different reasons people do it. There really are so many different types of people and reason as to why. Hopefully the stigma will go away soon. 3am here, time for some sleep. Take care.